The so called mixed connective tissue disease (MCTD), continues to be a controversial entity, while some authors considered it a good characterized disease, others think that is an undifferentiated connective tissue disease. OBJECTIVE. To analyse the clinical and serological evolution of a group of patients diagnosed of MCTD, with particular consideration to the meaning of anti-nRNP and anti-Sm antibodies. METHOD. We have studied 20 patients diagnosed of MCTD and 112 with systemic lupus erythematosus (SLE). Anti-nRNP and anti-Sm antibodies were detected through counter immunoelectrophoresis, immunoblotting and ELISA. RESULTS. After an average time of evolution of 10 years, 70% (14/20) of the patients diagnosed of MCTD fulfill criteria for SLE (6 cases), scleroderma (6 cases) or polymyositis (2 cases). Anti-nRNP response is persistent, directed mainly against the 70 Kd and A-nRNP polypeptides and qualitatively higher in MCTD in SLE (absorbencies 2.64 vs 1.25. The immunoblotting test detected anti-Sm antibodies in 5 patients (25%) and ELISA test in 14 (70%). CONCLUSIONS. Clinical and serological evolution suggest that MCTD is an undifferentiated connective tissue disease. Anti-nRNP antibodies are characteristic, although anti-Sm antibodies can be detected with ELISA regardless whether on not patients fulfill SLE criteria.